Re: The performance and behaviour of the anti-fragmentation related patches

[Christoph Lameter <clameter@engr.sgi.com>]

On Fri, 2 Mar 2007, Nick Piggin wrote:

> So what do you mean by efficient? I guess you aren't talking about CPU
> efficiency, because even if you make the IO subsystem submit larger
> physical IOs, you still have to deal with 256 billion TLB entries, the
> pagecache has to deal with 256 billion struct pages, so does the
> filesystem code to build the bios.

You do not have to deal with TLB entries if you do buffered I/O.

For mmapped I/O you would want to transparently use 2M TLBs if the 
page size is large.

> So you are having problems with your IO controller's handling of sg
> lists?

We currently have problems with the kernel limits of 128 SG 
entries but the fundamental issue is that we can only do 2 Meg of I/O in 
one go given the default limits of the block layer. Typically the number 
of hardware SG entrie is also limited. We never will be able to put a 



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